Interpretive Summary: Recently, there has been concern about increasing levels of greenhouse gases, such as carbon dioxide, in the atmosphere and the contribution of these gases to global warming. This has fostered an interest in ways to reduce atmospheric concentrations of carbon dioxide. Increasing the storage or sequestration of carbon dioxide in soil, in the form of organic carbon, has been suggested as a method for reducing atmospheric carbon dioxide. Plants fix carbon dioxide into organic carbon compounds through the process of photosynthesis; however, when this organic carbon is added to the soil bacteria and other microorganisms, soil can convert this organic carbon back into carbon dioxide through the process called respiration. Therefore, in order to better understand how soil carbon storage may be accomplished it is necessary to understand soil carbon dynamics. This study was done to measure the release of carbon dioxide via the respiration process from soil under corn/soybean agricultural crop production in central Iowa. Specifically, we investigated where soil respiration was dependant upon the type of soil and where the soil was located in the landscape. We found that landscape position was not a good predictor of how much carbon dioxide was released by soil repiration, but that the type of crop (either corn or soybeans) had a greater influence on soil respiration. These studies contribute to our understanding of soil carbon dynamics. This information will be useful to policy makers and farmers in deciding which types of agircultural management techniques will increase carbon dioxide storage in soils.

Technical Abstract:
Soil repiration is an important component of the carbon dynamics of terrestrial ecosystems. Many factors exert controls on soil repiration, including, temperature, soil water content, organic matter, soil texture, and plant root activity. This study was conducted to quantify soil respiration in the Walnut Creek Watershed in Central Iowa, and to invstigate the factors important in controlling this process. Six agricultural fields were identified for this investigation. Three of the fields were cropped with soybean (Glycine max (L.) Merr.) and three cropped with corn (Zea mays L.). Within each field soil respiration was measured at nine locations, with each location corresponding to one of the three general landscape element designations (summit, side-slope, and depression). Soil respiration was measured measured using a portable vented chamber connected to an infrared gas analyzer. Soil temperature measurements (5 cm depth) were performed and used to account for diurnal effects of temperature on respiration. Soil samples were collected at each location for measurement of soil water content, pH, texture, microbial biomass, and respiration potential. Field respiration rates did not show a significant landscape effect; however, there was a crop effect with respiration from the corn fields averaging 37.5 g CO2 m**-2 d**-1 vs. an average respiration of 13.1 g CO2 m**-2 d**-1 in the soybean fields. In contrast, the respiration potential of soils measured in the laboratory shows a significant landscape effect and an insignificant but cropping system effect. Similar relationships were observed for soil organic carbon and microbial biomass. These results indicate that plant root respiration in the field is a dominant component of soil respiration. In addition, correlation analyses indicate that corn roots may be more important that soybean roots in their contribution to surface CO2 flux.